A process wherein ammonium polysulfide is added to a sour water to convert cyanide was to thiocyanate ions has been described in U.S. Pat. No. 4,508,683 and in "Hydrocarbon Processing" (July 1981), p. 149-155. Those portions of the plant which are endangered by corrosion are supplied with an APS solution from a supply tank but there is no information how the APS solution is produced.